Description: Solid organ transplantation is the only curative treatment for end-stage kidney, liver, lung and heart failure but its success is limited b immunological rejection. Although immunosuppressive therapies are increasingly effective, their value is limited due to memory cell resistance to suppression, toxicities, and complications secondary to the general suppression of the host immune system, including opportunistic infections and malignancies. Rejection is initiated in humans when graft cells that express class I and class II MHC molecules, in particular endothelial cells that line the vasculature of the organ, are directly recognized by alloreactive memory CD8+ and CD4+ T lymphocytes, respectively. This project seeks to reduce rejection by altering the transplanted organ during the period around the time of transplant surgery when the host immune response is being shaped. I hypothesize that reducing MHC class II on graft endothelium will be sufficient to attenuate rejection responses by limiting the activation of CD4+ T cells, the principal orchestrators of graf rejection. I propose to accomplish this by using biodegradable nanoparticles to transfect graft endothelial cells ex vivo and deliver small interfering RNA (siRNA) that suppress the expression of the class II transactivator (CIITA), the master regulator of MHC class II expression. My specific aims are: (1) to characterize the effects of reducing MHC class II expression on cultured ECs by lipid-mediated transfection of anti-CIITA siRNA on modulating CD4+ and CD8+ alloresponses, as measured by cytokine production, proliferation and adoption of pathogenic phenotypes in vitro; (2) to characterize and optimize a nanoparticle delivery system that encapsulates anti-CIITA siRNA and is targeted to cultured human ECs by different ligands with an emphasis on maximizing the magnitude and duration of knockdown without inducing toxicity; and (3) to use optimized EC-targeted nanoparticles to transfect EC lining intact segments of human vessels and to assess the modulation of rejection of such vessels by allogeneic human peripheral blood mononuclear cells in a pre- clinical humanized mouse model.